Led Zeppelin IV further refined the band's unique formula of combining earthy, acoustic elements with heavy metal and blues emphases. The album included examples of hard rock, such as "Black Dog" and an acoustic track, "Going to California" (a tribute to Joni Mitchell). "Rock and Roll" is a tribute to the early rock music of the 1950s. In 2007, the song was used prominently in Cadillac automobile commercials—one of the few instances of Led Zeppelin's surviving members licensing songs.